1. Field of the Disclosure
The present disclosure relates generally to developing a magnetic brush for transferring toner to a photoconductive member, and particularly to selectively retracting the developed magnetic brush from contact with the photoconductor to substantially prevent background toner development.
2. Description of the Related Art
Electrophotography selectively develops toner to the photoconductor by discharging areas of the photoconductor that correspond to areas to be colored and leaving other areas charged so as to prevent toner development in those “white” areas. For negatively charged toner, these white areas are created by providing a photoconductor charge that is more negative than the developer bias. This difference between the photoconductor and developer biases is called the “white vector.” If the white vector is too small, then mechanical development takes place resulting in a haze of toner on areas of the photoconductor that should be white. Increasing the white vector produces a reduction in toner to the white areas, but if the white vector is increased too much there is another increase in toner developed into the white areas. This increased development is the result of high electric field strengths that can result in modifications to the toner charge. The white vector is adjusted to its desired value when the least amount of toner is developed into the white or background areas of the photoconductor. Toner developed into these white areas is called “background toner” and is wasted toner. This toner waste reduces the useful life of a toner cartridge since it represents an undesirable draw of toner for printing.
A typical laser printer will create an image on the page with approximately 0.4 mg/cm2 of print. Background toner with an appropriately adjusted white vector can be about 0.001 mg/cm2. This may seem like an insignificant amount of waste toner, but since the areas of unprinted “white” are typically orders of magnitude larger than the printed areas, this waste toner has greater significance. For color laser printers, the cost to the customer of background toner when printing a white page can, in some circumstances, approach or exceed the cost of a sheet of paper. Most laser printers use white vector to limit development of background toner when the machine is running but not actually developing toner that goes to the printed page. White vector is an effective way to keep the white areas of prints white, but an improved method is needed to prevent toner waste when not printing.